A process for removing nitrogen oxides from exhaust gases using a zeolite catalyst and a device for carrying out this process are known from DE-OS 3 642 018. As stated in detail therein, the zeolite catalyst, which may be in the form of a monolithic catalyst or loose-material catalyst, may preferably contain zeolites of the X and Y type or mordonite type. In this known process, the nitrogen oxides present in the exhaust gases of, for example, the internal combustion engine of a motor vehicle are reacted with the hydrocarbons also normally present in the exhaust gases as reducing agent and with the oxygen present during combustion of a lean fuel/air mixture. There is thus no need for the known addition of ammonia as reducing agent which is problematical for various reasons, particularly in motor vehicles.
Accordingly, the described effect of the known process presupposes a certain minimum concentration of hydrocarbons in the exhaust because, as described, they act as reducing agents. Accordingly, the known process cannot be optimally used at least in those cases where, as in diesel engines, the concentration of hydrocarbons in the exhaust gas is distinctly lower than the concentration of nitrogen oxides.